Figure 1.

The nucleotide cyclase wreath-like structure and active-site conformation. Comparison
of the C. reinhardtii guanylyl cyclase catalytic core (green) with that of the C1:C2 domains of activated
mammalian adenylyl cyclase (PDB ID: 1CJU; blue) bound to a nucleotide analog (shown as a stick model). The two active sites
of the guanylyl cyclase are found in a cleft formed at the interface of two identical
subunits (top left). The two subunits form a wreath-like structure, with active sites
located at either end of the cleft (bottom left). The nucleotide analog marks the
position of the single functional adenylyl cyclase active site. The guanylyl cyclase
structure is presumed to be in an inactive conformation: in the active state of the
nucleotide cyclases, it is expected that helix α1 (labeled in the right-hand panel)
moves into the active site and toward helix α4 of the other subunit of the dimer.
The arrows in the right-hand panel indicate the conformational changes required to
generate the active configuration. From Winger et al. [3] Figure 4 (PDB 3ET6).